This invention relates generally to a variable valve timing system for an internal combustion engine wherein the phase of an intake and/or exhaust camshaft(s) is adjusted in relation to the engine's crankshaft in order to achieve desired times of valve opening and closing during the engine cycle. More specifically, the invention relates to the autocalibration of camshaft phasing feedback for substantially eliminating discrepancies between apparent camshaft phasing, as measured by a feedback sensor, and the true camshaft phasing that exists by virtue of the mechanism that couples the camshaft to the crankshaft.
An example of a variable valve timing system that operates by adjusting the phasing of a camshaft, or camshafts, relative to the crankshaft is described in U.S. Pat. No. 4,744,338, commonly assigned. Camshaft adjustment is accomplished in a closed-loop manner by means of a mechanical adjustment mechanism that is operated by an electromechanical actuator. The electromechanical actuator is under the control of an electronic control unit (ECU). In order to assure faithful adjustment of the camshafts, a feedback sensor is associated with each camshaft under the ECU's control to provide camshaft phase feedback to the ECU. The closed-loop control that is provided by such feedback enables errors between the commanded phasing and the actual phasing to be nulled out assuming that the system is properly calibrated.
Due to sensor mounting errors, machine tolerances, engine-to-engine variations, etc., there is a certain amount of uncertainty in the cam phasing feedback, and this cannot be nulled out by the closed-loop control. Individual calibration of each sensor is a possible solution, but it is time consuming and expensive in a production application. The present invention relates to a solution that is significantly less time consuming and less expensive. The invention contemplates that the ECU itself will do the calibration with the resulting added benefit that such calibration will be on-going throughout the life of the system so that the effects of any changes in sensor-phasing relationship that occur after initial calibration will also be taken out of the system.
Where the ECU contains a microprocessor-based control, the invention can be readily implemented, mainly in software, although there is a mechanical aspect that must also be incorporated into the adjustment mechanism. In a system of the type contained in the aforementioned patent, the mechanism must be modified so that a short dwell is incorporated into the mechanism's adjusting cam. This dwell is located at a particular location on the cam profile such that when this dwell is acting on the cam follower that is associated with the cam, the camshaft phase is at a known relationship to the crankshaft. As the dwell acts on the cam follower, there is no resulting change in the camshaft phasing, and consequently no change in the sensor output will occur. The ECU detects this lack of phase change, and can therefore detect when the dwell is acting on the cam follower. Since the camshaft phase that exists when the dwell is acting on the cam follower is known, the ECU will know the actual, or true, camshaft phase at the same time that it is receiving the sensor feedback that is supposedly exactly representative of the true camshaft phasing. If the sensor feedback does not correspond to what it should be at this time, then the ECU will know that the sensor feedback is not properly calibrated. The ECU calculates the amount of the discrepancy by subtracting the actual sensor measurement from what it should be, and the result of this calculation becomes an offset that is algebraically added to the feedback from the sensor to create a properly calibrated feedback measurement. This autocalibration routine is programmed to occur at certain times during engine operation so that the accuracy of the feedback measurement is maintained. One way of programming the occurrence of the autocalibration routine is to initiate the routine whenever engine idle (closed throttle) is sensed. If the duration of any idle is sufficiently long, the routine will be completed. The offset is stored in memory associated with the microprocessor, but is updated after the completion of the autocalibration routine only if there is a change in the offset.
The foregoing features, advantages, and benefits of the invention, along with additional ones, will be seen in the ensuing description and claims which should be considered with the accompanying drawings. The drawings disclose principles of the invention according to the best mode contemplated at the present time for a preferred embodiment of the invention.